Canon 5D3 Flash Sync Problems

I returned bother units, one was defiantly not working, and bought 3 new ones. I am STILL having flash sync on the 5D3. Let me start with what I have as far as a setup. Canon 5D3. Cactus V6 (all 3 with new firmware) NO DELAY on any of them.. I have a Canon 600EX and a GODOX 860C. With the Cactus on the Canon 5D3, I get a slight dark area at the edge of the frame at 1/200. At 1/160, no frame. So, its defiantly seeing the shutter. When I use my other radios (phottix) at 1/200 NO dark band. NOW, here is the TOPPER. when I put a Canon 600 EX on the Cactus that is on the 5D3 and enable TTL pass thought, and turn the 600EX on (the one that is on the V6 that is ON the 5D3) 1/200 works FINE on the remote Canon 600EX and the remote Godox 860C. As soon as I turn the on-camera flash OFF, the 1/200 has a dark band.. Sometimes the band is tiny, other times it a little larger. I have tried this on 5 units, several flashes and 2 5D3.. Same problem..

When the 5D3 recognises a Canon flash -- or a Canon-dedicated trigger that pretends to be a Canon flash -- it changes the flash timing. It then supports 1/200s and potentially even higher sync-speeds.

When you set the V6 to TTL pass-through and have a Canon flash mounted on the V6 then the camera "sees" that flash and changes the flash timing.

In other words, I do not think that the problem is with the V6. You should see the same slight dark area with other fully manual triggers as well.

The only solution other than letting the camera "see" a Canon flash, could be to reduce the flash power which in turn will reduce the flash duration. This may cause the flash contribution to stop before the second curtain starts to close (and causes the dark area to develop).